STRUOTCTRE AND CLASSIFICATION OF THE ARTHROPODA. 539 



merome (e. g-. iipuromere, ninscle, or chitin-plnte) wlnVh cnn 

 be detected by minute observation (microscopic) as evidence 

 of their former existence. The somite of the maxillipede 

 (third post-oral appendage) of Apus c an cr if or mis is a good 

 example of a somite on its way to excalation. The third prte- 

 oral and the prasmaxillary somites of Hexapod insects are 

 instances whei'e the only traces of the vanished somite are 

 furnished by the microscopic study of early embryos. The 

 prasgenital somite of the Arachnida is an example of a somite 

 which is preserved in some members of the group and par- 

 tially or entirely excalated in other cases, sometimes with 

 fusion of its remnants to neighbouring somites. 



The Twelfth Law of metamerism might very well be 

 placed in logical order as the first. Itisthelaw of lipomer- 

 ism, and asserts that just as the metameric condition is 

 produced by a change in the bodies of the descendants of 

 unisegmental ancestors, so highly metamerised forms, i. e. 

 strongly segmented forms with specialised regions of differ- 

 entiated metameres, may gradually lose their metamerised 

 structure and become apparently and practically unisegmental 

 animals. The change here contemplated is not the atrophy 

 of terminal segments one by one so as to reduce the size of 

 the animal and leave it finally as a single somite. On the 

 contrary, no loss of size or of high organisation is necessary. 

 But one by one, and gradually, the metameric grouping of 

 the bodily structures disappears. The cuticle ceases to be 

 thickened in rings; the muscles of the body-wall overrun 

 their somite boundaries. Internal septa disappear. The 

 nei've-ganglia concentrate or else become diffused equally 

 along the cords ; one pair of renal coelomoducts and one pair 

 of genital coelomoducts grow to large size and remain — the 

 rest disappear. The appendages atrophy or become limited 

 to one or two pairs, which are widely dislocated from their 

 ancestral position. The animal ceases to present any indica- 

 tion of metameric repetition of parts in its entire structure. 

 Degrees in this process are frequently to be recognised. We 

 certainly can observe such a change in the posterior region 



